Frequency modulation apparatus and frequency modulation method

ABSTRACT

A frequency modulation apparatus capable of reducing a peak level of a radiation noise of a characteristic frequency band due to an image clock. This frequency modulation apparatus is used in an image formation apparatus having an image bearing body to be scanned by a laser beam, and comprises an auxiliary clock calculating portion for calculating an auxiliary clock period based on a reference clock period and a modulation coefficient, and an image clock generating portion for generating the image clock in which a frequency is different at least in one portion and other portions of an image area on a main scan line to be scanned by the laser beam on said image bearing body based on the initial period value set in advance and said auxiliary clock period, and said image clock generating means perform a frequency modulation so that the frequency of said image clock changes within a predetermined fluctuation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a frequency modulation apparatus and afrequency modulation method, which generate an image clock used for anon/off control of a laser beam scanning on a image bearing body such asa photosensitive drum and the like.

2. Related Background Art

In general, in an image forming apparatus of an electro-photographicsystem, while laser light emitted from a semiconductor laser is turnedon and off, this laser light is scanned by a polygonal rotating mirror(polygon mirror) so as to irradiate a photosensitive body, therebyperforming a latent image formation.

In such an image forming apparatus, an image clock of a constantfrequency is used for an on/off control of the laser light. The reasonwhy is because, if the frequency of this image clock is not constant,the on/off timing of the laser light deviates from a normal timing, andthereby a dot forming position of an electrostatic latent image formedon the photosensitive body is subtly displaced, and this results in theoccurrence of an image distortion, a color drift and a color shading.

Further, between the polygonal mirror and the photosensitive body, thereis provided a f-θ lens. This is because the f-θ lens has an opticalcharacteristic such as a corrective operation of a distortion aberrationwhich guarantees a converging operation of the laser light and timelinearity of a scan, thereby allowing the laser light having passedthrough the f-θ lens to be joint-scanned on the photosensitive body in apredetermine direction at an equal speed. However, due to a displacementof the characteristic of the f-θ lens, the laser light irradiated on thephotosensitive body is sometimes displaced from an ideal image formingposition. Hence, a frequency modulation technology is employed, in whichthis positional displacement of the image due to the f-θ lenscharacteristic is modulated to match a reference image clock so that theon/off timing of the laser light is subtly adjusted, thereby correctingthe position of the dot formed on the photosensitive body (for example,Japanese Patent Application Laid-Open Patent No. H2-282763).

However, when the image clock is always constant, there are often thecases where a radiation noise is generated and the level of theradiation noise exceeds the value defined in the international radiationnoise standard in the transmission path in which the on/off signal forturning on/off the laser light is transmitted from the generatingcircuit to a laser light driving circuit.

Further, when the frequency modulation technology is employed, thoughthe radiation noise level is reduced, in the case where the f-θ lenshaving a characteristic to such an extent that there is no need forperforming the frequency modulation is used, the image clock frequencybecomes constant, and therefore, the radiation noise level becomes muchsevere.

Particularly, in a color image forming apparatus of a tandem system andthe like in which the color drift in a main scan direction becomes aproblem, the frequency modulation is often used for correcting thecharacteristic of the f-θ lens, while in the color image formingapparatus of one drum system in which there is no need for being verysensitive about the color drift in the main scan direction or a blackand white image forming apparatus in which there is no need for takinginto consideration the color drift, the frequency modulation is scarcelyperformed, and even in that case, there are often the cases where theradiation noise level exceeds the value of the international radiationnoise standard, and this becomes a problem.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a frequency modulationapparatus capable of reducing a peak level of the radiation noise in acharacteristic frequency band due to an image clock.

In order to achieve the above-described object, a frequency modulationapparatus used in an image forming apparatus having an image bearingbody to be scanned by a laser beam, comprising: an auxiliary clockcalculating portion for calculating an auxiliary clock period based on areference clock period and a modulation coefficient; and an image clockgenerating portion for generating an image clock in which a frequency isdifferent at least in one portion and other portions of an image area ona main scan line scanned by the laser beam on the image bearing bodybased on an initial period value provided in advance and said auxiliaryclock period; wherein the image clock generating portion performs afrequency modulation so that the frequency of the image clock changeswithin a predetermined fluctuation range, and an image forming apparatushaving such frequency modulation apparatus are provided.

In such a configuration, since a frequency modulation is performed sothat an image clock frequency is changed within a predeterminedfluctuation range, the peak level of the radiation noise of thecharacteristic frequency band due to the image clock can be reduced.

Further, by performing the frequency modulation of the image clock,while controlling the influence due to a positional displacement of theimage to the minimum, the radiation noise level can be reduced with animage deterioration controlled to the minimum.

Further, by allowing a clock inputted to a PWM-IC to carry afluctuation, the radiation noise generated in the characteristicfrequency band of a conventional reference clock can be reduced, and ontop of that, an image forming can be performed without generating thepositional displacement of the image due to the fluctuation of thefrequency on the image.

The above and other objects, features, and advantages of the inventionwill become more apparent from the following detailed description takenin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically showing a configuration of an exposureunit of an image forming apparatus according to one embodiment of thepresent invention;

FIG. 2 is a block diagram showing a frequency modulation configurationof an image clock used in a driving control of a laser light source 1;

FIG. 3 is a graph showing a relationship between the image clockfrequency generated by the frequency modulation circuit configuration ofFIG. 2 and a main scan position;

FIG. 4 is a block diagram showing an internal configuration of afrequency control device 101 of FIG. 2;

FIG. 5 is a graph showing the relationship between a segment and aperiod of an image clock 18 within the segment;

FIGS. 6A and 6B are graphs showing the relationship when the period ofthe image clock 18 within the segment is varied at multi-steps;

FIG. 7 is a graph showing a change of the image clock frequency when asecond control method is performed;

FIG. 8 is a graph showing the change of the image clock frequency when afirst control method is performed;

FIG. 9 is a graph showing the change of the image clock frequency in amain scan direction when a yellow latent image is formed and a yellowtoner image is obtained;

FIG. 10 is a block diagram showing the frequency modulationconfiguration according to one embodiment of the present invention;

FIG. 11 is a graph shown about a generation of a frequency correctioncoefficient for a MCLK 106 and a MCLK 106 within a PWM-IC;

FIG. 12 is a block diagram illustrating a clock flow generated in thecircuit configuration of FIG. 10;

FIG. 13 is a block diagram showing the frequency modulationconfiguration according to another embodiment of the present invention;

FIG. 14 is a graph representation from a frequency fluctuation of aSSCG-CLK 111 and its conversion into a voltage value by an FV convertertill a generation of the frequency correction coefficient; and

FIG. 15 is a block diagram illustrating a clock flow generated in thecircuit configuration of FIG. 13.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail below withreference to the accompanying drawings showing a preferred embodimentthereof. In the drawings, elements and parts which are identicalthroughout the views are designated by identical reference numeral, andduplicate description thereof is omitted.

The embodiment of the present invention will be described below withreference to the drawings. FIG. 1 is a drawing schematically showing theconfiguration of an exposure unit of an image formatting apparatusaccording to one embodiment of the present invention.

An image forming apparatus of an electrophotographic system, as shown inFIG. 1, comprises an exposure unit which irradiates laser light on aphotosensitive drum 15 so that a latent image corresponding to aninputted image data is formed on the photosensitive drum 15. Thisexposure unit comprises a laser light source 1 emitting diffuse laserlight. The laser light emitted from the laser light source 1 isconverted into collimated laser light L1 through a collimator lens 13,and this laser light L1 is irradiated on a polygon mirror 2, which is inthe midst of a rotational driving, by a scanner motor 3. The laser lightL1 irradiated on a polygon mirror 2 is reflected by the polygon mirror 2and reaches a f-θ lens 14.

The laser light having passed this f-θ lens 14 is joint-scanned on thephotosensitive drum 15 in a main scan direction at an equal speed, andby a scan, that is, a scan operation of this laser light, the latentimage 16 is formed on the photosensitive drum 15. The start of the scanoperation of the laser light is detected by a beam detect sensor(hereinafter, referred to as BD sensor) 17. At the time corresponding tothe start of the scan of the laser light on the photosensitive drum 15,the laser light source 1 is compulsorily turned on, and the BD sensor 17detects the laser light reflected and inputted by the polygon mirror 2in a compulsory light-on period of the laser light source 1, and outputsa beam detect signal (hereinafter, referred to as BD signal) whichbecomes a reference signal of an image forming write timing for everymain scan.

Next, a frequency modulation configuration of the image clock used for adriving control of the laser light source 1 will be described withreference to FIG. 2. FIG. 2 is a block diagram showing the frequencymodulation configuration of the image clock used for the driving controlof the laser light source 1.

In the frequency modulation configuration of the image clock used forthe driving control of the laser light source 1, as shown in FIG. 2,there are provided reference clock generating means 20 for generating areference clock 21, a memory 63, and a frequency control apparatus 101.The memory 63 holds a frequency modulation setting parameter 56, and thefrequency modulation setting parameter 56 includes various types ofsetting values necessary for a modulating operation of the image clocksignal of the frequency control device (or apparatus) 101. Specifically,included are an image area start position setting value 57, an imagearea end position setting value 58, a frequency modulation setting valuebefore start of image area 59, a frequency modulation setting value inimage area 60, and a frequency modulation setting value after end ofimage area 61.

The image area start position setting value 57 is a value for setting aperiod (time) from the input of a BD signal 105 from the BD sensor 17 tothe image forming area start timing (image forming area start position)in a main scan direction. The image area end position setting value 58is a value for setting the period (time) from the input of the BD signal105 to the image forming area end timing (image forming area endposition) in the main scan direction.

Further, the frequency modulation setting value before start of imagearea 59 is a value for setting the image clock frequency generated bythe frequency control device 101 in the period (time) from the input ofthe BD signal 105 to an image area start timing (image forming areastart position) set based on the image area start position setting value57. The frequency modulation setting value in image area 60 is a value(value for setting a fluctuation amount of the image clock frequency)for setting the image clock frequency generated by the frequency controldevice 101 in an image area period (time) defined by the image areastart timing and the image area end timing set by the image area startposition setting value 57 and the image area end position setting value58. The frequency modulation setting value after end of image area 61 isa value for setting the image clock frequency generated by the frequencycontrol device 101 in the period (time) from the image area end timingset by the image area end position setting value 58 to the input of theBD signal 105 of the next main scan line. These various setting valuesare transmitted to the frequency control device 101.

The frequency control device 101 comprises segment dividing means 102for dividing an interior of one line which scans in the main scandirection into a plurality of segments constituted by the arbitrarynumber of pixels, and image clock generating means 103 for generatingthe image clock for a plurality of divided segments, respectively.Specifically, the image clock generating means 103 frequency-modulates areference clock 21 generated by reference clock generating means 104based on the BD signal 105 as well as the above-described varioussetting values, and generates an image clock 18. This generated imageclock 18 is inputted to a laser driving circuit 62, and the laser drivecircuit 62 drivingly turns on/off the laser light source 1 based on theinputted image clock 18 and an image signal.

Next, the image clock according to the frequency modulationconfiguration will be described with reference to FIG. 3. FIG. 3 is agraph showing the relationship between the image clock frequencygenerated by the frequency modulation circuit configuration of FIG. 2and a main scan position.

As shown in FIG. 3, as for the image clock of the period from the BDsignal 105 to the image area start (image area start position), theimage clock having a frequency sharply displaced against the frequencyof the reference clock 21 is used. This is because, in the area otherthan the image area, there is no need for the frequency which is correctand a constant clock since no image formation is made on thephotosensitive drum 15. In this way, the sharp displacement of afrequency in the above described period is effective for reducing thepeak level of the radiation noise in the characteristic frequency banddue to the image clock. Further, even in the period from the image endarea (END) to the inputting of the next BD signal, an image clock havinga frequency substantially different that of the reference clock issimilarly used. This is due to the same reason as applied to the periodfrom the BD signal 105 to the image area start (image area startposition), and is effective for reducing the peak level of the radiationnoise in the characteristic frequency band due to the image clock.

Next, a frequency setting in the image area as shown in FIG. 3 will bedescribed.

In the image area, when the image clock is used, in which a frequency isdisplaced more than necessary against the reference clock 21, thereoccurs a displacement in which the image forming position on thephotosensitive drum 15 is displaced from a normal position (idealposition). However, when the image clock is put into a state, in whichthere hardly exists a fluctuation, though the displacement of the imageforming position disappears, it is not possible to reduce the peak levelof the radiation noise in the characteristic frequency band due to theimage clock.

Hence, in the present embodiment, a method for generating the imageclock is used in which the reference clock 21 is frequency-modulated sothat image deterioration due to the displacement of the image formingposition is controlled to be at or below 10 μm (unrecognizable by nakedeyes). That is, in order that the displacement of a local image formingposition due to the frequency modulation being performed more thannecessary is controlled within a range difficult to confirm by the nakedeye and also the radiation noise level due to the image clock isreduced, the image clock is generated, in which the image clockfrequency has the fluctuation within the frequency range of ±2% or less,for the frequency of the reference clock 21. The fluctuation amount ofthis image clock is set for every segment which is divided by thesegment dividing means 102 based on the frequency modulation settingvalue in image area 60.

Further, it is preferable that a fluctuation component is, as shown inFIG. 3, set so that a main scan magnification fluctuation of the imagedue to a fluctuation component of the frequency in the image area periodis set off by the image area period. In this way, the main scanmagnification fluctuation of the whole image is not generated. However,though a variable-magnification of the write position is slightlygenerated due to the fluctuation component between the image areaperiods, since this is a level visually unseeable (insensitive), theimage deterioration is not invited.

Here, an internal configuration of the frequency control device 101 andthe image clock generated by the same will be described below withreference to FIGS. 4, 5 and 6. FIG. 4 is a block diagram showing theinternal configuration of the frequency control device 101 of FIG. 2,FIG. 5 a graph showing the relationship between the segment and a periodof the image clock 18 within the segment, and FIG. 6 a graph showing therelationship when a period of the image clock 18 within the segment isvaried at multi-steps.

The frequency control device 101, as shown in FIG. 4, comprises avariable-magnification coefficient setting register 22, an auxiliarypixel generating circuit 24, an initial period setting register 26, amodulation clock control circuit 30, a number of pixels setting register31, and a modulation clock generating circuit 28, and these circuits andregisters constitute segment dividing means 102 and image clockgenerating means 103.

The variable-magnification coefficient setting register 22 stores avariable-magnification coefficient 23 for varying a period ratio of thereference clock 21 generated from reference clock generating means 104.The auxiliary pixel generating circuit 24 generates an auxiliary pixelperiod 25 based on the reference clock signal 21 and thevariable-magnification coefficient 23. By this auxiliary pixel period25, a main scan magnification is corrected. That is, since a dot widthor a dot interval of the main scan on the photosensitive drum 15 doesnot become uniform due to the optical system of the polygon mirror 2 andthe f-θ lens 14 of FIG. 1, the correction of the image clock frequencyin one scan period is performed using the auxiliary pixel period 25 sothat the dot width or the dot interval becomes uniform. For example, inthe case of a rotational scan system such as the polygon mirror 2, bothend portions in the main scan direction of the photosensitive drum 15tend to be fast in a scan speed, and in reverse, a central portion inthe main scan direction of the photosensitive drum 15 tends to be slowin the scan speed. Hence, by speeding up the image clock frequency inthe vicinity of both end portions of the photosensitive drum 15 andcorrecting the image clock frequency of the central portion of thephotosensitive drum 15 to be slow, it is possible to allow the dot widthor the dot interval on the photosensitive drum 15 to be uniform.

Here, in the frequency control device 101, it is possible to executeeither one of a first control method for dividing one main scan lineinto a plurality of segments and generating a constant image clock 18for every segment or a second control method for performing thefrequency modulation of the image clock within each of the dividedsegments.

First, the first control method for dividing one main scan line into aplurality of segments and generating a constant image clock 18 for everysegment will be described with reference to FIG. 5.

For example, where the period of the reference clock signal 21 is takenas τref, the variable-magnification coefficient 23 as αk, and a periodof the auxiliary pixel period 25 as Δτ, Δτ is represented by thefollowing expression (1):Δτ=αk·τref   (1)

Here, the variable-magnification coefficient 23 (=αk) is set to such avalue that the period Δτ becomes sufficiently shorter than the period ofthe image clock 18.

The initial period setting register 26 stores an initial value 27 (τvdo)of the period of the image clock 18 outputted from the modulation clockgenerating circuit 28.

A modulation clock control circuit 30 divides the interior of one line,which scans in the main scan direction, into the segments constituted bythe arbitrary number of pixels and forms a plurality of segments. Basedon the frequency modulation setting value in image area 60, themodulation clock control circuit 30 keeps control so that the imageclock period has a predetermined range of fluctuation within eachsegment. The number of pixels within the segment is set by a number ofpixels setting value 32 within the number of pixels setting resister 31.The number of pixels between each segment may be either the same ordifferent number.

Here, the detail of the operation of the modulation clock controlcircuit 30 will be described. The modulation clock control circuit 30generates a modulation clock control signal 33 for an initial segment(segment 0) when the BD signal 105, which is outputted from the BDsensor 17 and becomes a write reference, is inputted, and outputs thecontrol signal 33 to the modulation clock generating circuit 28. Themodulation clock generating circuit 28 which receives this modulationclock control signal 33 outputs the image clock 18 of the initial period27 (τvdo)

For the next segment (segment 1), the modulation clock control circuit30 generates the modulation clock control signal 33 for the next segment(segment 1), and outputs the control signal 33 to the modulation clockgenerating circuit 28. The modulation clock generating circuit 28 whichreceives this modulation clock control signal 33 generates a modulationclock signal ΔT1 having a period represented by the following expression(2) as the image clock 18 based on the auxiliary pixel period 25 and theinitial period 27 (τvdo)ΔT1=τvdo+α·τref   (2)

Here, α is a variable modification coefficient for the segment 1.

Similarly, further for the next segment (segment 2), the modulationclock control circuit 30 outputs the modulation clock control signal 33for the next segment (segment 2) to the modulation clock generatingcircuit 28. The modulation clock generating circuit 28 which receivesthis modulation clock control signal 33 generates a modulation clocksignal ΔT2 having the period represented by the following expression (3)as the image clock 18 based on the auxiliary pixel period 25 and theinitial period 27 (=τvdo):ΔT2=τvdo+α·τref+β·τref   (3)

Here, β is a variable modification coefficient for the segment 2.

Further, even in the case where more segments are available after thesegment 2, modulation clock signals are generated for those segmentsavailable by the similar procedure, and are outputted as the image clock18.

As described above, by the control of the modulation clock controlcircuit 30, the image clock 18 having a plurality of periods in theinterior of one main scan line is outputted from the modulation clockgenerating circuit 28.

Next, the second control method for performing the frequency modulationof the image clock within each segment will be described with referenceto FIGS. 6A and 6B.

In the case where the frequency of the image clock 18 is varied from theinitial segment (segment 0), as shown in FIG. 6A, when an initial periodis taken as τvdo, the number of pixels for every one segment as n, amodulation coefficient (segment 0) as α, and a reference clock period asτref, the period Δτa for every one pixel at the segment 0 and the totalperiods ΔT0 of the segment 0 are represented by the followingexpressions (4) and (5):Δτa=(α·τref)/n   (4)ΔT0=τvdo+{n·(n+1)/2}·{(α·τref)/n}=τvdo+{(n+1)/2·(α·τref)}  (5)

In the case where the frequency of the image clock 18 of the initialsegment (segment 0) is fixed and the frequency of the image clock 18 ofsubsequent segments is varied, as shown in FIG. 6B, when the totalperiods of the segment 0 are taken as ΔT0, they are expressed by thefollowing expression (6):ΔT0=n·τvdo   (6)

On the other hand, for the next segment of the initial segment, that is,the segment 1, when the modulation coefficient (segment 1) is taken asβ, and the reference clock period as τref, the period Δτb for every onepixel at the segment 1 and the total periods ΔT1 of the segment 1 areexpressed by the following expressions (7) and (8):Δτb=(β·τref)/n   (7)ΔT1=τvdo+{n·(n+1)/2}·{(β·τref)/ n}=τvdo 30 {(n+1)/2}·(β·τref)   (8)

With respect to each segment after that, the period Δτb for every onepixel and the total periods ΔTn (n≧2) of each segment can be expressedby the similar expression.

Next, the change of the image clock frequency will be specificallydescribed with reference to FIG. 7, where the above described secondcontrol method is performed. FIG. 7 is a graph showing the change of theimage clock frequency when the second control method is performed. Thisdrawing is enlarged in a part of the image area so that a frequencysetting particularly in the image area can be easily understood.

First, the frequency modulation setting value in image area 60 withinthe memory 63 is transmitted to the frequency control device 101. Here,the frequency modulation setting value in image area 60 includes thefrequency setting value for every segment, and the segment dividingmeans 102 divides the image area into a plurality of segments accordingto the number of frequency setting values included in the frequencymodulation setting value in image area 60. In the present embodiment, apredetermined count interval, in which the image clock is set as a countvalue, is taken as one segment.

As shown in FIG. 7, in a segment a, the image clock generating means 103modulates and controls the frequency so that the frequency setting valueb1 is connected from the frequency setting value a1 by a straight line.Subsequently, in a segment b, the image clock generating means 103modulates and controls the frequency so that the frequency setting valuec1 is connected from the frequency setting value b1 by the straightline. For the subsequent segments, the similar control is made. In thisway, by controlling the frequency for each segment, the image clock isallowed to carry a fluctuation for the reference clock. Hence, the peaklevel of the radiation noise in the characteristic frequency band due tothe image clock can be reduced.

Next, the change of the image clock frequency when the above-describedfirst control method is performed will be specifically described withreference to FIG. 8. FIG. 8 is a graph showing the change of the imageclock frequency when the first control method is performed.

In the case of the present embodiment, the segment is set similarly withFIG. 7. In the case of the present embodiment, though the frequencysetting value within the segment is constant, a different frequency isset between each segment so that the reference clock is allowed to carrya fluctuation. In this way, the peak level of the radiation noise in thecharacteristic frequency band due to the image clock can be reduced.

Next, in the color image forming apparatus, the image clock frequencysetting in the main scan direction will be described with reference toFIG. 9, where a yellow latent image is formed and a yellow toner imageis obtained. FIG. 9 is a graph showing the change of the image clockfrequency in the main scan direction when the yellow latent image isformed and the yellow toner image is obtained.

In this case, a modulation ratio of the frequency in the image area islarge against other colors, and a displacement amount of the image iscontrolled so as to become 15 μm or less. The reason why is because,when a color image is obtained in an electrophotographic color imageforming apparatus, it is prevalent to obtain a full color image bysuperposing toner images of four colors of Yellow, Magenta, Cyan andBlack so as to form an image. At this time, the color drift by thedisplacement of a yellow image position is wide in latitude comparing toother colors, and while the other colors are visually recognizable at 10μm in the color drift, in the case of Yellow color, it is possible tovisually recognize the color drift only at 15 μm. Hence, in the presentembodiment, by making the most of the above-described visualcharacteristic during a yellow image formation, a fluctuation isprovided for the frequency for the frequency setting in the main scanimage area within an allowable range of generating a positionaldisplacement of the image up to 15 μm. For example, during the yellowimage formation, the image clock frequency in the main scan direction ischanged as shown in FIG. 9. Hence, during the yellow image formation,the radiation noise level of the characteristic frequency band generateddue to the image clock can be further reduced.

In this way, in the present embodiment, since the frequency modulationis performed so that the image clock frequency is changed within apredetermined fluctuation range, the peak level of the radiation noiseof the characteristic frequency range due to the image clock can bereduced.

Further, in the color image forming apparatus of one drum system inwhich the performance of the f-θ lens is too good to perform thefrequency modulation or there is no need to be very sensitive about thecolor drift in the main scan direction, or again in the black and whiteimage forming apparatus in which there is no need to be sensitive aboutthe color drift, there is scarcely any need to perform the frequencymodulation. Even in such a case, though there have been many cases wherethe radiation noise level exceeds the international radiation noisestandard, in the present embodiment, by performing the frequencymodulation of the image clock while controlling the influence due to thepositional displacement of the image to the minimum, it is possible toreduce the radiation noise level with the image deterioration controlledto the minimum.

Further, it is possible to constitute the configuration including thewhole or a part of the block which forms the above-described frequencycontrol device 101 or the configuration including a block in thevicinity thereof as ASIC or other integrated circuits.

Next, the frequency modulation configuration of the image clock used inthe driving control of the laser light source 1 will be describedfurther in detail with the reference to FIG. 10. FIG. 10 is a blockdiagram showing the frequency modulation configuration of the imageclock used in the driving control of the laser light source 1, and isone example of the circuit in the image forming apparatus of four-drumsystem in which four sets each of the photosensitive body and exposuremeans, that is, four configurations of FIG. 10 are mounted in tandem.

In the frequency modulation configuration of the image clock used in thedriving control of the laser light source 1, as shown in FIG. 10, thereare provided the reference clock generating means 20 for generating thereference clock 21 which becomes a reference to keep a constantfrequency, an image control circuit 100, and PWM-IC 104 a to 104 d. Inthe image control circuit 100, there are provided the frequency controldevice 101 and the DELAY measuring means 103.

Note that the internal configuration of the frequency control device 101and the image clock generated by the same are same as described above.

The frequency modulation configuration of the image clock used in thedriving control of the laser light source 1 will be described withreference to FIG. 10. Among the BD signals 105 a, 105 b, 105 c and 105 dinputted to the image control circuit 100, and this BD signal isinputted as the BD signal 105 of FIG. 4, and generates the MCLK 106generated by the first frequency control device 101 shown in FIG. 2. Inorder to reduce the radiation noise in the characteristic frequency banddue to the reference clock, the MCLK is a clock in which a fluctuationis provided in the frequency by carrying a specific period located inthe reference clock 21, and a fluctuation amount of the specific periodand the frequency is set in advance in a register of the frequencycontrol device 102, and according to that set value, the reference clockis modulated and outputted.

The DELAY measuring means 103 inputs the BD signals 105 a, 105 b, 105 cand 105 d obtained respectively from the BD sensor 17 of four exposuredevices, and measures a DELAY amount of other BD signals for the BDsignal 105 a which becomes the reference BD signal of the frequencycontrol device 101, and transmits the measuring result to three PMW-IC104 b to 104 d, respectively.

The PWM-IC 104 a to 104 d generates image clock IMG_CLK 107 a to 107 dbased on the measuring result from the inputted MCLK 106 and each BDsignal 105 a to 105 d and the DELAY measuring means 103. A frequencydecode value memory located within the PWM-IC 104 a to 104 d holds acorrection coefficient for decoding a frequency setting set in thefrequency control means 101 of the image control circuit 100 andapplying a frequency correction to the same so that it becomesapproximately the reference clock 21. Based on each BD signal and thedelay amount from the reference BD signal 105 a, outputted from saidDELAY measuring means, a data read address is displaced by the delayedportion of the reference BD signal 105 a stored in the frequency decodevalue memory, so that, by this correction coefficient, it is possible togenerate a clock approximately close to the reference clock 21 by thefrequency control device mounted also inside the PWM-IC.

Further, the PWM-IC 104 a to 104 d are mounted with a f-θ characteristiccorrection value memory for correcting a characteristic irregularity ofthe f-θ lens mounted in each exposure unit, and by further correctingthe characteristic of this f-θ lens to the image clock approximatelydecoded by the reference clock, it is possible to align the imageforming position on the photosensitive body with a high accuracy.

FIG. 11 is a graph about the MCLK 106 generated in FIG. 10 and thegeneration of the frequency correction coefficient for the MCLK 106within the PWM-IC of FIG. 10.

FIG. 12 is a block diagram illustrating a clock flow generated in thecircuit configuration of FIG. 10. The reference clock (REFCLK) 21inputted from a reference clock input 200 is modulated to a frequency201 synchronized with the BD signal, and generates the MCLK 106. Thegenerated MCLK 106 is decoded 202 into the frequency approximately closeto the reference clock within the PWM-IC, and based on this clock 205,the correction coefficient of the f-θ characteristic is further appliedand the frequency is modulated 203, thereby obtaining the IMG_CLK 107.

Next, another embodiment of the present invention will be described withreference to FIG. 13. FIG. 13 shows a circuit configuration in anotherembodiment of the present invention.

First, the reference clock 21 having a constant frequency generated bythe reference clock generating means 20 is inputted to a spread spectrumclock generator 110 (hereinafter, abbreviated as SSCG). This SSCG 110 isa device which adds a fluctuation to the frequency of the inputtedreference clock 21, and this allows the radiation noise in the frequencyrange due to the reference clock 21 to be reduced. The SSCG_CLK 111generated by this SSCG 110 is inputted to PWM-IC 104 a, 104 b, 104 c and104 d provided for the laser of respective exposure units. Here, byusing the PWM-IC 104 a, the internal circuit configuration will befurther described. First, within the PWM-IC 104 a, the SSCG-CLK 111 isinputted to an FV converter 112 a and converts the frequency into avoltage value. From this voltage value, the frequency is measured to seehow much it is displaced form the reference clock, and based on thismeasurement result, a correction coefficient is decided by correctioncoefficient calculating means 113 a so that the displaced portion fromthe reference clock is corrected, and the correction coefficient ismultiplied by the SSCG_CLK 111 in the frequency control device 101within the PWM-IC 104 a, so that a clock approximately equal to thereference clock 21 can be obtained. Further, here also, similarly withthe embodiment of FIG. 10, the PWM-IC 104 is further mounted with a f-θlens characteristic correction value memory 108 a for correcting thecharacteristic irregularity of the f-θ lens mounted on each exposureunit, and by correcting the characteristics of the f-θ lens further tothe image clock approximately decoded to the reference clock, it ispossible to align the image forming position on the photosensitive bodywith a high accuracy.

FIG. 14 is a graphical representation of the fluctuation of thefrequency of the SSCG_CLK 111 in FIG. 13, and a process in which thefluctuation is converted into the voltage value by the FV converter soas to generate the frequency correction coefficient.

Further, FIG. 15 is a block diagram illustrating the flow of the clockgenerated in the circuit configuration of FIG. 13, and the referenceclock (REFCLK) 21 inputted from the reference clock input 200 isfrequency-modulated 204 by the SSCG and generates a SSCG_CLK 111. Thegenerated SSCG_CLK 111 is decoded 202 into the frequency approximatelyclose to the reference clock within the PWM-IC, and based on that clock205, the correction coefficient of the f-θ characteristic is furtherapplied so as to perform the frequency modulation 203, thereby obtainingan IMG_CLK 107.

In this way, by allowing the clock inputted to the PWM-IC to carry thefluctuation by the circuit configurations of FIGS. 10 and 13, theradiation noise generated in the characteristic frequency band of theconventional reference clock can be reduced, and on top of that, theimage formation can be made without generating the positionaldisplacement of the image due to the fluctuation of the frequency on theimage.

Further, in FIGS. 10 and 13 of the present embodiment, though the colorimage forming apparatus of the four drum system engine having fourexposure units is cited as an example, and the system which lines upfour PWM-ICs is described, in the present invention, needless tomention, it is a technology which can be put to practical use whether itis the image forming apparatus of one drum system having one PWM-IC orthe image forming apparatus having a plurality of more than four drums.

1-6. (canceled)
 7. A frequency modulation apparatus used in an imageforming apparatus having an image bearing body to be scanned by a laserbeam, comprising: a segment dividing portion for dividing a main scanline scanned by said laser beam on the image bearing body into aplurality of segments each constituted by a plurality of pixels; anauxiliary clock calculating portion for calculating an auxiliary clockperiod based on a reference clock period and a modulation coefficient;and an image clock generating portion for generating image clocksrespectively corresponding to the plurality of segments based on aninitial period value provided in advance and said auxiliary clockperiod; wherein said reference clock is a clock having a predeterminedfrequency fluctuation, said auxiliary clock calculating portioncalculates the auxiliary clock by setting the modulation coefficientcorresponding to said respective segment, so as to reduce a fluctuationof said reference clock, and said image clock generating portiongenerates an image clock so as to reduce the fluctuation of thefrequency of said reference clock based on said auxiliary clock.
 8. Thefrequency modulation apparatus according to claim 7, wherein thefluctuation of the predetermined frequency of said reference clock is afluctuation ±3% or less for the clock frequency having no fluctuation.9. The frequency modulation apparatus according to claim 7, furthercomprising a frequency variation device for generating the fluctuationof the predetermined frequency of said reference clock, wherein saidfrequency variation apparatus generates said reference clock having thefluctuation of the predetermined frequency by adding a constant clock tothe fluctuation of the predetermined frequency by carrying a certainpredetermined period for a constant clock which becomes the reference.10. The frequency modulation apparatus according to claim 9, whereinsaid frequency variation device has measuring means for measuring afluctuation component of the frequency of the reference clock generatedby said frequency variation device, and based on that result, decidesthe modulation coefficient corresponding to said respective segment. 11.The frequency modulation apparatus according to claim 9, wherein theperiod of the fluctuation of the reference clock generated by saidfrequency variation device has a period synchronized with a intervalsignal of the main scan line, and by setting a variation coefficient ofthe frequency of each segment according to the period of the fluctuationof said reference clock, generates the image clock in which thefluctuation component of the frequency of said reference clock isreduced by adding and subtracting said auxiliary clock to and from saidreference clock.
 12. (canceled)